(Fig. 20) with an elastic base, but with glass-like, brittle, rounded
heads, which break off at the lightest touch, whereupon the sharp
point of the broken hair penetrates the skin of the creature which has
touched it, and the poisonous contents of the hair are poured into
the wound. Even our large stinging-nettle (_Urtica dioica_) can cause
intense irritation, and evoke the 'nettle-rash,' named after it, on
the human skin; but there are many tropical species of nettle, e.g.
_Urtica stimulata_ in Java, and others, which have an effect similar
to that of snake-poison and produce tetanoid spasms, and so on. In
addition to formic acid these hairs contain an undefined ferment, a
so-called Enzyme. It need scarcely be said that these stinging-hairs
must have much more severe effects on the mucous membrane of the mouth
of grazing animals than on the human skin, and that they are therefore
an excellent protection for the plants. As a matter of fact we never
find our nettle patches eaten away, and even the donkey, which eats
thistles freely, turns away from the stinging-nettle. But even these
stinging-hairs, like all other protective devices, do not afford an
_absolute_ protection. The caterpillars of several of our diurnal
butterflies feed exclusively on the stinging-nettle, and they eat up
the leaves, stinging-hairs and all. This is the case with five species
of the genus _Vanessa_, namely: _Vanessa io_, the 'peacock,' _Vanessa
urticæ_, the small tortoiseshell, _Vanessa prorsa_, _Vanessa C. album_,
the C. butterfly, and _Vanessa atalanta_, the admiral.
[Illustration: FIG. 19. Barbed bristles of _Opuntia rafinesquii_;
enlarged.]
[Illustration: FIG. 20. Vertical section through a piece of a leaf of
the Stinging-nettle _(Urtica dioica_), bearing two stinging-hairs;
magnified 85 times; adapted from Kerner and Haberlandt.]
We are all familiar with our mulleins (_Verbascum_), those beautiful
flower-spikes with the thick, soft felt leaves, which grow on stony or
sandy soil. Harmless as they look, they are much disliked by animals
as food, for the thick hairy felt which covers them breaks up in the
mouth, and sticks in the folds of the mucous membrane, causing burning
sensations and other discomforts. They, too, are therefore spared by
grazing animals, but they have smaller enemies, like the caterpillars
of the genus _Cucullia_, which, however, never completely destroy them,
but only eat large holes in their leaves.
Let us now consider in somewhat greater detail the true thorns, the
most conspicuous protection of many plants. It is very remarkable that
these are always so placed, and so regulated as to their length and
character, as to afford protection to the most important and the most
exposed parts of the plant. Thus many bushes, which would otherwise be
in danger of being completely devoured by cattle, are stiff with thorns
which are nothing else than pointed, hard twigs without, or with very
little foliage. Among these are the sloes, the buckthorn (_Rhamnus_),
the sea-buckthorn (_Hippophäe_), and the barberry (_Berberis_). In the
last-named three thorns arise in a group, and protect the young bud
from danger in three directions (Fig. 21).
[Illustration: FIG. 21. A piece of a twig of Barberry (_Berberis
vulgaris_) in spring; after Kerner.]
The fine-leaved mimosas of the tropics have similar but very long and
sharp thorns, and their leaves are movable and sensitive, so that, when
they are touched, they shut up and draw back behind the rampart of
stiff thorns, which are just of the right length to protect them.
In many thorny bushes only the young shoots of each spring remain
green through the summer, and in autumn they become transformed into
thorns, under whose protection the shoots of the following spring will
develop. Sometimes, too, the leaf-stalks are modified in the course of
the summer into thorns, as in Tragacanth (_Astragalus tragacantha_).
In this case the young leaves are protected by a circle of thorns,
consisting of the leaf-stalks of the preceding year which have not
fallen off (Fig. 22, _A_, _B_, _C_).
I should have to go on for a long time with my exposition, even if I
were to confine attention to the essential facts; we shall, therefore,
only recall the well-known phenomenon of the Cactuses, in which the
leaves are entirely transformed into spines, which may attain a length
of eight centimetres, while the fleshy stem alone represents the
green--that is, the assimilating parts of the plant. The species of
Cactus are almost the only plants which grow on the stony, hard, and
hot plateaux of Mexico, and they are protected from desiccation by the
thickness of their epidermis. But, enticing as is the food promised
by the juicy stem, animals rarely venture to approach them, and it is
only when tortured by thirst that horses and asses occasionally knock
off the spines with their hoofs, and so reach the soft tissues rich in
water. For this attempt, however, as Alexander von Humboldt pointed
out, they often suffer, as the sharp spines are apt to pierce the hoof.
In any case, the cactuses are effectively protected from the danger of
extermination by grazing animals.
[Illustration: FIG. 22. Tragacanth (_Astragalus tragacantha_). _A_,
two spring shoots. _B_, a single leaf, from which the three uppermost
leaflets have fallen off. _C_, leaf midrib, from which all the leaflets
have fallen off. After Kerner.]
It must certainly strike every one that many districts, especially
those which are dry, hot, and stony, are conspicuously rich in thorny
plants, and it has often been supposed that the production of thorns
must be a direct result of these peculiar conditions of life; indeed,
the hard, thorny habit of many of these plants has even been regarded
as a protection against desiccation. This, however, is contradicted
by all those thorny plants which, like the cactuses, possess tissues
extremely rich in sap, and in which desiccation is prevented, not
by the thorns, but by the thick epidermis. The only satisfactory
explanation is that afforded in terms of natural selection. In such
hot, and at the same time dry regions, the plant-growth is often very
scanty, and the food available for the grazing animals is, at least at
times, very scarce; on this account, if the plants are to survive there
at all, they must be armed with the most perfect means of protection
possible against the attacks of hungry and thirsty animals. The
struggle for existence in relation to such enemies is much more severe
than in more luxuriant regions, and the protection by thorns has been
developed to the highest possible pitch of perfection; species which
were unable to develop this protection died out altogether. Hence the
cactuses of Mexico, and the many thorny bushes and shrubs of the hot,
and, in the summer, dried-up stony coast-lands of the Mediterranean in
Spain, Corsica, Africa, and other countries. This so-called 'Prigana
scrub' embraces a number of species, whose nearest relatives in our
climate are not provided with spines, as, for instance, _Genista
hispanica_, _Onobrychis cornuta_, _Sonchus cervicornus_, _Euphorbia
spinosa_, _Stachys spinosa_, and others.
Why do so few thorny plants grow on the rich and well-watered Alpine
pastures? Probably because there is to be found there a rich and
luxuriant plant-growth which can never be wholly exterminated by
the grazing of animals, so that an individual species would not, by
developing thorns, have gained any advantage in the way of increased
capacity for existence.
But these Alpine grazing grounds serve well to illustrate how great
may be the advantage which protective devices give to a species.
Much to the annoyance of the herdsmen, who endeavour to extirpate
them as far as possible, enormous masses of rhododendrons often
cover whole stretches, because their hard silicious leaves cannot be
eaten, and many other plants despised of cattle flourish and increase
on the grazing runs, like the repulsively bitter, large _Gentiana
asclepiadea_, the malodorous _Aposeris fœtida_, and various ferns of
disagreeable taste.
The advantage derived by plants from the possession of any kind of
protective device against grazing animals is perhaps best of all seen
in the 'shrubbery,' which on every Alp is to be found in the immediate
neighbourhood of the herdsman's hut. There, where the cattle daily
assemble, and where the soil is continually being richly manured by
them, we always find a large, luxuriantly growing company of the
poisonous aconite, the bitter goosefoot (_Chenopodium bonus henricus_),
the stinging-nettle, the thistle (_Cirsium spinosissimum_), the
ill-smelling _Atriplex_, and some other inedible species, while the
palatable herbs are gradually exterminated by the cattle which daily
gather round the hut (Kerner).
To sum up. We have seen that there is among plants an extraordinary
diversity of protective adaptations, which secures them from
extermination by the larger herbivores.
Since all useful contrivances, or, as we say, all adaptations, are
capable of interpretation in terms of the process of selection, we
must refer this great array of the most diverse protective devices
to natural selection; and again, as among animals, we receive the
impression that the organism is, to a certain extent, really capable
of producing every variation necessary to its maintenance. Literally
speaking, this would not be correct, but at any rate the number of
adaptations possible to each form of life must be an enormous one, so
great, indeed, that ultimately every species does secure protection
for itself in some manner and in some degree, whether it be by the
production of a poison or a nauseous substance within itself, or by
surrounding itself with thorns or spines. And if it be, in a certain
sense, a matter of 'chance' whether a plant has taken to one method of
defence or to another, according as its innate constitution favoured
the production of one rather than of any other, yet it would not
be easy to prove, even in the case of the purely chemical means of
protection, that these would have occurred in the same distribution
and concentration as a necessary result of the metabolism of the
plant, even if they had not been useful and consequently augmented
by selection. But in the case of the mechanical means of protection
this mode of explanation fails as utterly as that of the direct
effect of the conditions of life. Why the holly should have spinose
leaves beneath and smooth ones above can never be deduced from the
constitution of the species.
While the protective adaptations of plants against the larger
herbivores always point to natural selection, our appreciation of the
adaptability of plants, and at the same time of the potency of natural
selection, will be strengthened still more if we turn our attention for
a little to the arrangements which prevent the extermination of plants
by the lower and small animals.
It might indeed be supposed that extermination by these could hardly
be an imminent danger, but if we think of the cockchafer blight, or of
the destruction of whole woods by the caterpillar of the 'white nun,'
or even of the destruction of several successive plantings of young
salad plants which the snails often cause in our gardens, it cannot be
doubted that all plants would be exterminated by insects and snails
alone unless they were protected against them in some degree.
We owe our detailed knowledge of the means by which plants protect
themselves against the menace of the greedy and prolific snails to
the beautiful investigations of Stahl, Professor of Botany in the
University of Jena.
In this case, too, both chemical and mechanical means are made use of.
The minute quantity of tannic acid which is contained in the leaves of
the clover prevents many snails from eating them, as, for instance,
the garden snail (_Helix hortensis_). If the leaves be soaked so as
to wash out the tannin the snail readily accepts them as food. It is
true that the small, whitish field-slug (_Limax agrestis_) does not
object to the presence of the tannin, and eats the fresh leaves of
the clover; indeed, there is no such thing as absolute protection.
In discussing the herbivorous mammals I have already mentioned that
many trees and shrubs, mosses and ferns are effectively protected by
the large amount of tannin they contain; this protection is effective
also against snails, for all these plants are fairly free from their
attacks; and the same is true of many other tannin-containing plants,
species of saxifrage and sedum, the strawberry, many water-plants, like
the pond-weeds (_Potamogeton_), the horn-nut (_Trapa_), the mare's tail
(_Hippuris_). All these plants are only eaten by snails in case of
necessity, or in the washed-out state.
In other plants protection is gained by means of some acid, especially
oxalic acid, like the wood-sorrel (_Oxalis acetosella_), the sorrel
(_Rumex_), and the species of Begonia. When Stahl smeared slices of
carrot, which is a favourite food of snails, with a weak (one per
cent.) solution of oxalate of potassium, they were refused by the
snails, and this is not surprising when we remember that even the
external skin of the snail is very sensitive, and the mucous membrane
of the mouth is not likely to be less so.
Similarly, many plants develop ethereal oils in the hairs which cover
them, as in the herb-Robert (_Geranium robertianum_). Even the almost
omnivorous field-slug (_Limax agrestis_) does not touch this plant, and
if it be placed upon it, escapes with all dispatch from the ethereal
oil, which burns its naked skin, by covering itself with mucus and
letting itself down to the ground by a thread. The mints (_Mentha_) and
the dittany (_Dictamnus albus_) also produce such oils.
Among chemical means of protection must be named the pure bitter
stuffs, such as are found in the species of gentian, the milkwort
(_Polygala amara_), and in many other plants, and also the curious
'oil-bodies' of the liverworts.
But some plants also defend themselves against the attacks of snails by
mechanical means.
First there are the various kinds of bristle arrangements, which
prevent the snails from creeping up the stalks. We never find the
comfrey (_Symphytum officinale_) of our meadows eaten by snails,
for it is thickly covered over with stiff bristles, which are most
disagreeable to the snail, and the stinging-nettle (_Urtica dioica_) is
similarly protected by bristle hairs, while, as we have already seen,
its stinging-hairs secure immunity from the attacks of larger animals.
And although it is true that the majority of plants do not prevent the
snails from creeping up their stalks, yet they do not serve them in any
great degree as food, since the green parts often offer resistance to
mastication and digestion. Thus the lime encrustations which cover the
stoneworts (_Chara_) prevent snails from eating them. If the lime be
dissolved by means of acids, and the plants then offered to the snails,
they will eat them greedily. The same is true of the silicifying of
the cell-walls, so widely distributed among mosses and grasses, and
when this occurs in a high degree it forms an effective protection
even against the large herbivores. Our slightly siliceous grasses are
secure from snails, and that it is really the presence of the silicic
acid which deters them from an otherwise welcome kind of food is proved
by Stahl's experiment of growing maize in pure water, and so obtaining
plants poor in silica. These were devoured without ceremony by the
snails.
Of the many other protective peculiarities which make it difficult for
snails to eat plants I shall only recall the so-called 'Raphides,'
those microscopic crystal-like needles of oxalate of lime, pointed
at both ends, which lie close together in the tissues of many
plants. Cuckoo pint (_Arum maculatum_), the narcissi, the snowdrops
(_Leucojum_), the squill (_Scilla_), and the asparagus contain them,
and all these plants are spared by snails obviously because during
mastication they are unpleasantly affected by the raphides. Even the
voracious field-slug rejects these.
Of course it cannot be said that these raphides protect against all
other enemies. They are effective against rodents and ruminants,
and also against locusts, but a number of caterpillars seek out by
preference just those plants which contain raphides. Thus certain
caterpillars of the Sphingidæ feed on species of _Galium_ and
_Epilobium_, the leaves of the vine, and the wild balsam (_Impatiens_).
The caterpillar of _Chærocampa elpenor_, which especially prefers
_Vitis_ and _Epilobium_, has transferred its affections to the fuchsias
in our gardens, which came from South America; the butterfly not
infrequently lays its eggs on these plants, and the caterpillars devour
them readily; but the fuchsias may also contain raphides.
We may say, indeed, that almost all wild Phanerogams are protected in
some degree against snails, and this almost suggests the question: What
then is left for the snails to feed on if everything is thus armed
against them? But, in the first place, there remain our cultivated
plants, which, like the garden lettuce (_Lactuca_), are quite without
defence; and secondly, the snails often eat the plants only after
they have been rooted up and lie rotting on the ground, that is,
when the protective ingredient has been dissolved out by the rain;
finally, no means of protection, as I have often said already, is
absolute or effective against all snails. Many of these are, as Stahl
calls them, 'specialists.' Thus, the large slug of our woods eats the
poisonous fungi which are rejected by other snails, and in the same
way there are many other specialists which, however, are not likely to
eliminate unaided the plants to which they have adapted themselves.
There are certainly also omnivorous forms, like the field-slug
(_Limax agrestis_), to which we have referred so often, and _Arion
empiricorum_, the red slug, but just because these eat so many kinds of
plant they are less dangerous to any one species.
These manifold devices for protecting plants against the depredations
of snails afford another proof that innumerable details in the
organization of plants, as of animals, must be referred to natural
selection, since they are capable of interpretation in no other way.
If these protective devices were to be found only in isolated plants,
we might perhaps talk of 'chance'; we might refer them to the inborn
constitution of the plant, which made the production of bristles, or
bitter stuffs, or the deposition of silicic acid a necessity, and which
'happened' to make the plants distasteful to certain snails. But as
it appears that all plants are protected against snails, one in this
way, another in that, this objection cannot be sustained. Furthermore,
some of the beautiful experiments made by Stahl to prove the protective
effect of these devices showed, at the same time, that they were not
in themselves indispensable to the existence of the plant; maize, for
instance, develops a plant perfectly capable of life, even though
silicic acid be withheld, and the acid is, therefore, not an element
essential to its constitution, but a means of protection against
voracious animals. The clearest proof of this is afforded by plants
like the lettuce (_Lactuca_), which formed protective stuffs in the
wild state, but have lost them altogether under cultivation, through
disuse, as we shall see more precisely later on. As the eyes of animals
which live in darkness have degenerated, so the plants which have been
taken under the protection of man have lost their natural means of
defence, because these were no longer necessary to the maintenance of
the species. Even the protective bitter substances (tannin-compounds)
are not essential to the constitution of the genus _Lactuca_; their
formation may be discontinued without the plant being otherwise
affected. And in this case it is not a question of the withdrawal
of something which has to be taken in from outside, it is the
non-development of what is purely a product of the internal metabolism.
The adaptations of plants against snails are instructive in another
way, namely, in their extraordinary diversity. Here again we see how
great is the plasticity of organic forms, and how precisely, though
in many very different ways, they adapt themselves to the conditions
of their life, in this case the weaknesses of their greedy enemies,
and all to attain the same end, the security of their existence as a
species. We see at the same time that innumerable minute details in the
structure and character of a species, which may appear unimportant, may
yet have their definite uses--hairs, bristles, and raphides, as well as
bitter substances, ethereal oils, acids, and tannin-compounds. But we
must, of course, have minute and exhaustive investigations, like those
of Stahl, in regard to the biological relations of these peculiarities
before their utility can become clear to us.
LECTURE VII
CARNIVOROUS PLANTS
Introduction--The Bladderworts or Utriculariæ--Pitcher-plants,
Nepenthes--The Toothwort, Lathræa--The Butterwort, Pinguicula--The
Sundew, Drosera--The Flytrap--Aldrovandia--Conclusions.
THAT the principle of selection dominates, to a large extent at least,
all the structural characters of plants, and moulds these in direct
relation to the prospects of greater success which may be offered in
the vicissitudes of the life-conditions of a single species or group
of species, is nowhere more apparent than in the case of the so-called
'insectivorous' or 'carnivorous' plants. Here again it was Charles
Darwin who led the way, for while many plants had long been known on
the sticky leaves of which insects were often caught and killed, it
had occurred to no one to regard this as of any special use for the
plant, much less to look on the peculiar dispositions of such leaves
as especially determined for this purpose. Darwin was the first to
show that there is no small number of plants--we now know about
500--which secure only a portion of their nutritive material by the
usual method of assimilation, and gain another and smaller portion by
dissolving and utilizing animal protoplasm, especially nitrogenous
muscle substance. The correctness of this interpretation was at first
disputed, but Darwin showed that pieces of muscle, or any nitrogenous
organic substance, were really dissolved by the relevant parts of the
plant, and were afterwards absorbed. It can therefore no longer be
doubted that the remarkable contrivances by which animals are laid
hold of by plants--are in a certain sense caught and killed--have
arisen with reference to this particular end; or, to speak less
metaphorically, that existing structural and functional peculiarities
in a plant which caused animals to be held fast were of advantage to
the nutrition of the plant, and were therefore augmented and perfected
by natural selection. That this was possible is obvious from the number
of insectivorous plants which now live upon the earth, and that these
processes of selection ran their courses quite independently of one
another, and even that they started from different parts of the plant,
is shown by the diversity of the contrivances which occur in plants of
several different families. A few of these I wish to discuss in some
detail.
[Illustration: FIG. 23. _Utricularia grafiana_, after Kerner. _A_, a
plant in its natural position, floating in the water. _FA_, traps. _B_,
a trap enlarged four times. _sz_, suctorial cells. _kl_, valve, which
closes the entrance to the trap. _C_, suctorial cells on the internal
wall of the trap, enlarged 250 times.]